Information and telecommunications networks including the Internet as a representative thereof have been widely deployed all over the world as an infrastructure essential to people's lives. As technologies supporting the traffic of the Internet, there have been optical communication technologies each utilizing an optical fiber. An optical communication device employing a silicon platform, which can deal with a band of 1.3 μm and a band of 1.55 μm among optical fiber communication wavelength bands, has been expected as one which makes it possible to realize a high-density optical integrated circuit by utilizing a complementary metal oxide semiconductor (CMOS) fabrication technology.
As a particularly important technology among such optical communication technologies, there is an optical waveguide device fabrication technology. An optical waveguide device makes it possible to, unlike an optical device fabricated by splicing discrete optical parts, integrate a large number of optical devices. Accordingly, a design flexibility regarding optical devices is significantly improved. Moreover, fabrication technologies for semiconductor devices can be utilized, and thus, low-cost and large-scale-integrated devices can be achieved.
Examples of such an optical waveguide device include an optical splitter, an optical coupler, a wavelength multiplexer/de-multiplexer, an optical switch, an optical modulator and a variable optical attenuator. Nevertheless, there has been a problem that, when stray light or leakage light occurs inside such an optical waveguide device, it gives adverse effects on the characteristics of the optical waveguide device, such as an extinction ratio, cross talk and the like. Moreover, not only for a single device, but also for a device including a large number of optical devices integrated therein, there has been a problem that stray light or leakage light from a device gives adverse effects on the characteristics of another optical device.
As a specific example of such a case, an optical switch realized by employing a Mach-Zehnder interferometer is exemplified here. As shown in FIG. 13, this optical switch, which is constituted by a Mach-Zehnder interferometer, turns on/off of each of light rays outputted from output ports 1306a and 1306b by causing a directional coupler 1302 to split a light ray from an input port 1301 into two light rays, causing a phase shifter 1304 to control a light phase difference between light rays having passed through two arms 1303a and 1303b, and then, causing a directional coupler 1305 to merge the light rays having passed through the two arms into a light ray again. At this time, although, normally, there is no input and output light at a dummy port 1307, leakage light from the dummy port 1307 of a device is likely to arise when a light reflection point has arisen inside the device for some reason.
There arises a problem that re-coupling of this leakage light with the waveguide degrades the characteristics of the optical device, such as an extinction ratio and cross talk, and thus, it becomes necessary to completely quench light at the dummy port 1307 in some way.
Among optical terminators having been proposed so far, there is an optical terminator which is structured such that a slot with an angle relative to a waveguide is provided at an edge portion of a dummy port, and afterwards, this slot is filled with an optical absorption material, such as carbon black (refer to PTL 1 and PTL 2). However, this method makes fabrication processes therefor more complex because of additional processes of forming a slot, filling the formed slot with an optical absorption material, and the like, and further, the fabrication processes therefor becomes more difficult because of the occurrence of exfoliation thereof and the like. Thus, this method results in high cost, and further, problems in reliability, a variation of performance and the like have been significant problems.
Further, it has been proposed to fabricate a structure for scattering light at an edge portion of a dummy port (PTL 3). In this proposal, nevertheless, there has been a significant problem that this scattering of light results in the occurrence of stray light, so that this stray light gives adverse effects, such as the lowering of an extinction ratio and the occurrence of cross talk, on other optical devices. Further, even if light can be scattered not within a face on which an optical device is mounted but in an upward and downward direction relative to the face, the scattered light is likely to be coupled with an optical receiver and the like installed in a package containing the optical device, and thus, this has been a significant problem.
Further, there has been proposed a structure which allows light to leak from a core included in a waveguide connected to a dummy port by causing the core to be of a forward tapered shape, and allows a metallic material or the like covering the surrounding of the core to optically absorb the leaked light (PTL 4). In this technology, nevertheless, there has been a problem that conditions for a fabrication process of causing the core included in the waveguide to be of the forward tapered shape are so severe that it is difficult to fabricate such a taper of the forward tapered shape.